Geochemical characteristics and geological significance of desorbed shale gas in Longmaxi Formation, Jiaoshiba area

doi:10.7623/syxb201607003

Acta Petrolei Sinica ›› 2016, Vol. 37 ›› Issue (7): 846-854.DOI: 10.7623/syxb201607003

• Petroleum Exploration • Previous Articles Next Articles

Geochemical characteristics and geological significance of desorbed shale gas in Longmaxi Formation, Jiaoshiba area

Qin Hua, Fan Xiaojun, Liu Ming, Hao Jingyu, Liang Bo

Sinopec Exploration Branch, Sichuan Chengdu 610041, China

Received:2015-12-15 Revised:2016-05-23 Online:2016-07-25 Published:2016-07-28

焦石坝地区龙马溪组页岩解吸气地球化学特征及地质意义

秦华, 范小军, 刘明, 郝景宇, 梁波

中国石油化工股份有限公司勘探分公司四川成都 610041

通讯作者: 秦华,男,1980年1月生,2004年获中国地质大学(武汉)学士学位,现为中国石油化工股份有限公司勘探分公司工程师,主要从事石油地质综合研究。Email:qinh.ktnf@sinopec.com
基金资助:
中国石油化工股份有限公司科技部项目"四川盆地及周缘区块下组合页岩气形成条件与有利区带评价研究"（P13129）资助。

Abstract

Abstract:

This study is focused on the carbon isotope reversal of shale gas in Silurian Longmaxi Formation, Sichuan Basin. Core desorption test is conducted on the shale in Longmaxi Formation, Jiaoshiba area, so as to analyze its gas components and carbon isotope compositions. The results indicate that the desorbed gas is obviously wetter than the wellhead gas in component, and the carbon isotope of the former is significantly heavier; the carbon isotope values of each component became greater with the increase of desorption time. In terms of different samples, the maximum variation of δ¹³C₁ value is from 12.3 ‰ to 23.9 ‰ , while that of δ¹³C₂ is only from 0.8 ‰ to 2.3 ‰ , i.e., the carbon isotope of methane presents more obvious variations than heavy hydrocarbon, which is consistent with the results of previous shale core desorption experiments. It is concluded that δ¹³C₁ value observed in core desorption process has more obvious variations than δ¹³C₂ value, which is not caused by the difference in diffusion rate, but mainly results from the differences in desorption stages, i.e., ethane is in its early desorption stage while methane in the late desorption stage. In the production process, the differences in the component phase of alkane hydrocarbon gas and those in desorption stage caused by adsorption-desorption process may be the major reasons for complete reversal of the carbon isotopes of shale gas in Longmaxi Formation, Sichuan Basin. However, it cannot be denied that the mixing of kerogen pyrolysis gas and crude oil cracking gas make a partial or even a major contribution to the reversal of carbon isotopes in shale gas.

Key words: reversal of carbon isotope, carbon isotope fractionation, desorbed gas, shale gas, Jiaoshiba area, geological significance

摘要：

通过对焦石坝地区龙马溪组页岩岩心进行解吸以分析其气体组分和碳同位素组成，研究了四川盆地志留系龙马溪组页岩气碳同位素倒转现象。结果表明，解吸气相对井口气组分明显偏湿、碳同位素值明显偏重；各组分碳同位素值随解吸时间变重：不同样品δ¹³C₁值最大变重幅度12.3 ‰ ~23.9 ‰ ，而不同样品δ¹³C₂值最大变重幅度仅0.8 ‰ ~2.3 ‰ ，即甲烷碳同位素值相对重烃变化更明显，与前人页岩岩心解吸实验结果一致。研究结果认为：地层状态下页岩气可能并未发生碳同位素倒转，岩心解吸过程中观察到的δ¹³C₁值比δ¹³C₂值变化更明显，不是不同组分扩散速率差异造成，而主要是由于甲烷与乙烷处于不同解吸阶段导致，即乙烷处于其解吸早期阶段而甲烷处于其解吸较晚阶段；生产过程中吸附作用引起的烷烃气不同组分相态差异与所处解吸阶段差异可能是导致四川盆地龙马溪组页岩气碳同位素完全倒转的主要原因，但不能否认干酪根裂解气与原油裂解气的混合对页岩气碳同位素倒转做出的部分甚至大部分贡献。

关键词: 碳同位素倒转, 碳同位素分馏, 解吸气, 页岩气, 焦石坝地区, 地质意义

CLC Number:

TE122.1

Qin Hua, Fan Xiaojun, Liu Ming, Hao Jingyu, Liang Bo. Geochemical characteristics and geological significance of desorbed shale gas in Longmaxi Formation, Jiaoshiba area[J]. Acta Petrolei Sinica, 2016, 37(7): 846-854.

秦华, 范小军, 刘明, 郝景宇, 梁波. 焦石坝地区龙马溪组页岩解吸气地球化学特征及地质意义[J]. 石油学报, 2016, 37(7): 846-854.

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Geochemical characteristics and geological significance of desorbed shale gas in Longmaxi Formation, Jiaoshiba area

焦石坝地区龙马溪组页岩解吸气地球化学特征及地质意义

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